Abstract

Fogo is the largest of the three active central volcanoes on São Miguel and dominates the centre of the island. It is located at the intersection of NW–SE, NE–SW and east–west-trending fault systems, showing a complex morphology with a Summit Caldera formed as a result of explosive and collapse events. The edifice of Fogo has been extensively dissected by erosion, with deep valleys that show clear tectonic control.

The products of Fogo range from basalt to trachyte and belong to a potassic alkaline suite. The oldest subaerial products of Fogo are >200 ka. Older products are poorly exposed, making stratigraphic correlation difficult, particularly on the north flank where considerable subsidence within the NW–SE Ribeira Grande graben has occurred. A more complete stratigraphy for the last 40 ka was established on the southern flank of the volcano. During this period there were large trachytic Plinian eruptions, including those of the Ribeira Chã (8–12 ka BP) and Fogo A (4.6 ka BP). The last intracaldera eruption was historic and occurred in 1563, and 4 days afterwards there was an effusive basaltic eruption on the northwestern lower flank of the volcano.

Fogo Volcano, also known as Água de Pau, is the highest of the three active central volcanoes of São Miguel (Fig. 8.1), rising to an altitude of almost 1000 m with a summit caldera that is partially occupied by Fogo Lake. In the last 10 ka Fogo has been characterized by major trachytic explosive activity of which two events, Ribeira Chã (8–12 ka BP) and Fogo A (4.6 ka BP), were of Plinian scale, generating pyroclastic flows that reached the coast. Significant tephra fall deposits were also produced. The most recent eruption from the summit occurred in 1563 and, although sub-Plinian in scale, still deposited material over much of the eastern side of the island. Fogo is important, not only because of the opportunity it provides to improve understanding of trachytic volcanism, but also because of the hazard it poses to the population of São Miguel. Even when dormant, Fogo and the neighbouring Congro Fissural Volcanic System are affected by frequent seismic swarms (Silva 2004; Wallenstein et al. 2009; Silva et al. 2012). The largest historical earthquake that has affected São Miguel took place on 22 October 1522, with an epicentre near to Vila Franca do Campo (Machado 1959; Silveira et al. 2003). It triggered two massive landslides that flowed over the site of the village (Marques 2004; Wallenstein et al. 2007). The Fogo massif (Fig. 8.2) is covered by vegetation and is extensively mantled by volcaniclastic materials. This surficial geology, combined with heavy precipitation, favours the generation of frequent landslides, which flow along deeply dissected, structurally controlled valleys (Marques et al. 2006). High infiltration rates on the higher flanks of the volcano feed an important hydrothermal reservoir heated by the magma chamber, and this is exploited for geothermal power by a plant installed on the volcano's northern flank.

Central volcano and calderas

The morphology is strongly influenced by tectonic structures. The northern sector of the volcano has been down-faulted by a NW–SE-trending graben (Fig. 8.4). The subaerial portion of the edifice rises to 947 m asl at Pico da Barrosa, occupies an area of approximately 132 km2 (Fig. 8.4) and has a total volume of about 44 km3. The Summit Caldera (the inner caldera), formed by ‘scars’ produced by multiple explosions and collapses and subsequently widened by erosion, occupies an area of approximately 4.8 km2 and has a maximum diameter of about 3.2 km (Fig. 8.4). The caldera is occupied by a lake, the Lagoa do Fogo, having an area of about 1.5 km2, and three small lagoons, one of which is frequently dry (Fig. 8.5). The lake occupies the deepest part of the depression with the walls of the caldera ranging in height from 370 m above the lake – near Pico da Barrosa – to <10 m in the southern sector.

To the north of the Summit Caldera, a broad concentric erosion pattern is found accompanied by trachytic domes (Fig. 8.4). This pattern is related to the presence of the remains of an older eroded caldera wall (the Outer Caldera; Moore 1991a, b). One of the principal streams that dissects the volcano, the Ribeira Grande, separates the Summit and Outer calderas within the Lombadas valley and here CO2-rich mineral water springs are found.

One of the distinctive aspects of Fogo is the dominance of an erosional morphology. This is particularly noticeable in the higher areas, where deeply cut valleys form a dense drainage network. The outer northern and southern flanks have been extensively dissected by deep valleys that are usually tectonically controlled.

By analysing flank profiles it is possible to distinguish two slope patterns with an average slope of 3° in the lower part and varying from 13° to 24° at higher altitudes. The change coincides with the presence of massive trachytic lava flows and domes that form the conical summit of the edifice. The slopes at lower elevations are smoothed by thick sequences of pyroclastic deposits.

The coastline shows great diversity. Some areas have cliffs >100 m in height, whilst other parts are dominated by lava flows that are only a few metres thick. Several types of volcanic construct are observed on Fogo and these are described in turn below.

Scoria cones

Scoria cones occur on the flanks of the massif and are aligned NW–SE, suggesting tectonic control (Fig. 8.4). They represent monogenetic structures that resulted from the accumulation of basaltic scoria produced during Hawaiian and Strombolian eruptions. A basaltic spatter rampart formed during the 1563 basaltic eruption on the summit of the older trachytic dome of Pico do Sapateiro.

Pumice cones

Pumice cones are usually monogenetic structures but can have a wide variety of sizes. In the Fogo massif five monogenetic pumice cones were identified: one inside the Summit Caldera; and the others on the northern flank, one in Mata do Botelho Peak, two in Coroa da Mata and one at the west end of Santa Bárbara beach.

Hydrovolcanic forms

Hydromagmatic volcanic activity forms tuff cones and tuff rings that typically have a low height and large bowl-shaped craters. Two hydrovolcanic landforms occur inside the Summit Caldera as remains of tuff rings or tuff cones. One of these is associated with the 1563 and 1564 eruptions. In the sea, south of Vila Franca do Campo, a small islet with the same name as the village is a well-preserved tuff cone with an almost circular-shaped crater. Remnants of hydromagmatic deposits located along the northern coast of the Fogo massif in the Calhau do Cabo area may indicate the former existence of a tuff ring or tuff cone that has since been washed away by the sea.

Lava flows and domes

Owing to the compositional variety of magmas and their range of viscosities, a wide variety of forms and structures have been produced. Lavas range from basalts through to more evolved trachytes. Basaltic lava flows predominantly show aa morphology and occur in the low-lying area between Ribeira Seca and Ribeirinha and in the lava deltas of the southern coast and in the bottom of some valleys. Pahoehoe lava flows that fill some narrow valleys also occurred, for example, during the 1563 eruption of Pico do Sapateiro. At higher levels on the flanks of the central volcano, inside the calderas as well in the northern and southern coastal regions, thick trachytic lava flows occur, many of which have a dome-like morphology.

The application of the parameters defined by Blake (1989), based on the relationships between the ratio and height during the development of an andesitic dome on Soufrière (St Vincent in 1979), led to a selection of the 19 domes of Fogo massif that returned different results, with a steeper trend line and lower correlation values. A more detailed analysis allowed us to separate the Fogo domes into two subsets by size. The smaller domes produced a trend line very similar to that obtained by Blake (1989), while the larger domes produce a much steeper trend and an even higher correlation (0.98). These results indicate that the statistical relationships established by Blake (1989) might not be applicable across a wide range of dome sizes and this finding needs testing on other volcanoes.

Tectonic structures

The spatial distribution of eruptive centres and drainage systems, together with aerial photographic interpretation, was the main tool used in identifying the principal tectonic structures of Fogo (Wallenstein 1999; Wallenstein et al. 2007).

The location of Fogo Volcano may be associated with the intersection of several structural directions, as has been suggested for the Azorean volcanoes as a whole (Gaspar 1996; Queiroz 1997). The Azores region's principal tectonic regime (Carmo et al. 2015; Madeira et al. 2015; Miranda et al. 2015) is reflected on the northern flank of Fogo (Fig. 8.4) with a predominant NW–SE to NNW–SSE direction, this being well defined by the alignment of scoria cones and lava domes, the direction of valleys and the distribution of seismic activity (Silva et al. 2012, 2015). It may also be inferred by major discontinuities in electrical resistivity (Gandino et al. 1985). This NW–SE direction corresponds to faults defining the so-called Ribeira Grande graben, a major distensional structure associated with downfaulting of the order of 650 m (Muecke et al. 1974). This structure corresponds to a fault system that crosses the Fogo Volcano Summit Caldera. On the southern flank an important set of structures with clear north–south direction is reflected in the direction of the valleys (Fig. 8.4). The existence of alignments of NE–SW direction is also supported by Gandino et al. (1985) in observed discontinuities in electrical resistivity.

The distribution of the trachytic domes in the north appears to be aligned with the faults of the Ribeira Grande graben, whereas just to the north of Lombadas the domes are arranged in a curved, circumferential manner that possibly reflects a circular tectonic structure associated with the Outer Caldera. For a more comprehensive analysis of tectonic features on Fogo reference should be made to Carmo et al. (2015).

Fogo Volcano stratigraphy

The first work on volcanic stratigraphy of Fogo was published by Walker & Croasdale (1971), who studied the two major eruptions from the volcano that have occurred during the last 5 ka: Fogo A and 1563. Later, with Basil Booth, George Walker and Ron Croasdale extended this work to the volcanism of São Miguel over the last 5 ka (Booth et al. 1978). In the 1970s and during preparatory work associated with the exploitation of the geothermal reservoir, many reports and other works were published. Moore (1986, 1990) produced the first stratigraphic study of the volcano as a whole, later including a 1:50 000-scale geological map of São Miguel (Moore 1991a), which was supported by an extensive programme of radiometric dating (Moore & Rubin 1991). More recently a comprehensive stratigraphy of the last 40 ka was established by Wallenstein (1999) for the southern flank of the volcano. On the northern flank, several eruptive series have been documented, but it has proved possible to correlate only deposits from the last 4.6 ka with those on the southern flank. New road exposures allowed Pimentel (2004) to make some tentative correlations between older deposits on the north and south flanks.

Fogo Volcano has a complex eruptive history stretching over >200 ka (Muecke et al. 1974; Moore 1990, 1991a, b) and the close vicinity of the Picos and Congro fissural volcanic systems and Furnas Volcano leads to the intercalation of deposits from different volcanoes on the lower flanks of Fogo.

Wallenstein (1999) proposed a stratigraphic scheme comprising two major units: the Lower Group and the Upper Group. The Lower Group includes all the products older than 40 ka. This constitutes most of the volume of the Fogo massif and these deposits are not differentiated in the present work because they are poorly exposed. The Upper Group incorporates all of the volcanics erupted since the formation of the Outer Caldera. Moore (1991a, b) places the age of the Outer Caldera between 46 and 26 ka, and associates its formation with the eruption of large volumes of welded and non-welded ignimbrite. A date of 34.2 ka BP (Shotton & Williams 1973, in Booth et al. 1978) has been obtained for a unit near the base of the Upper Group and this, together with a date of >40 ka (Moore & Rubin 1991) for the lavas underneath its deposits, would support an age of around 40 ka ago.

Lower Group

This unit consists of volcanic materials that predate the formation of the Outer Caldera. The oldest date obtained for material from the volcano, but with a high degree of uncertainty, is 280±140 ka. This date was determined from submarine basaltic lava collected at a depth of 950 m in a 981 m borehole drilled in 1973 on the lower northern flank of the volcano (Muecke et al. 1974; McGraw 1976). The presence of submarine lavas in this stratigraphic position marks the period before which the edifice emerged as an island. According to these authors, three thick eruptive sequences occur above the pillow lavas, each showing evidence of magmatic differentiation with a transition from basaltic lavas to progressively more explosively related deposits with pumice horizons found at the top of each sequence. Most of the volcanics described in the core (Muecke et al. 1974) we consider to belong to the Lower Group. About 78% of the material is lava, essentially alkaline basalts, hawaiites, mugearites and a small volume of trachytes. The remaining 22% are pyroclastic deposits, with the majority being of trachytic composition (McGraw 1976).

The oldest dated outcrop of Fogo Volcano belongs to the trachytic dome of Eira Velha, located near the sea on the volcano's southern flank. It is dated at 181±15 ka BP (Gandino et al. 1985; Moore 1991a, b). Trachytic lava flows and domes are observed in the sea cliffs between Ribeirinha and Porto Formoso, as well as in some sparse outcrops located on the higher central part of the volcano and in the Summit Caldera walls. These include a lava flow that crops out at an altitude of around 750 m and is dated at 121±5 ka (Gandino et al. 1985; Moore 1991a, b). This may be correlated with a period of the volcano's history that is represented by a sample of trachytic lava collected at a depth of 57 m within the drill core. It has been dated at 117±5 ka (Muecke et al. 1974).

Important sequences of volcaniclastic deposits include tephra and thick ignimbrites that are sometimes interbedded with lava flows and are also of trachytic character. A welded tuff at the base of one of these sequences, located in a sea cliff east of Santa Iria harbour, was dated by Gandino et al. (1985) at 103±7 ka.

Upper Group, pre-5 ka

The history of the volcano for the last 40 ka is better understood. There is a bimodal distribution of volcanism, with activity occurring both on the flanks of the edifice and from the Summit Caldera. On the southern flank an ignimbrite towards the base of the Roida da Praia Formation has been dated by Shotton & Williams (1973, in Booth et al. 1978) at 34.2 ka, and ignimbrites occur on the northern flank that are older than 21 ka (Wallenstein 1999). Although Wallenstein (1999) and later Pimentel (2004) have characterized many deposits associated with Fogo, the difficulty in establishing stratigraphic relations north to south led them to define two separate eruptive sequences, respectively for the southern and northern flanks of the volcano. A third grouping includes all the basaltic products distributed on both flanks of the volcano. Wallenstein (1999) established a continuous sequence on the southern flank but could identify a correlation with the northern flank for only the last 5 ka, based on the Fogo A unit (Walker & Croasdale 1971). Pimentel (2004) has, however, tentatively correlated a sequence exposed on the northern flank with the Ribeira Chã Formation that has been defined on the southern flank. Ponte (2013) established new correlations of deposits previously described on the northern flank, between Ribeirinha and Maia.

Owing to difficulties in correlating many of the deposits in some eruptive sequences or even in deciding whether they are all related to Fogo, we classified only those formations that clearly belong to Fogo's geological history. The distinctive Ribeira Chã and Fogo A formations are used as principal stratigraphic marker horizons. Similarly, we describe deposits from basaltic eruptive activity associated with low dispersion flank eruptions using Fogo A and products of dated historical eruptions as stratigraphic references.

The identified eruptive sequences pre-5 ka are described from the north and south flanks separately (see Fig. 8.6). The most comprehensive stratigraphic scheme can be determined for the southern flank.

Northern Flank

Porto Formoso eruptive sequence

This comprises a large sequence of pyroclastic deposits, which is >25 m thick. It occurs between Ladeira da Velha and Moinhos beach and consists mainly of pumiceous materials that were produced by several eruptions. These episodes were separated by time intervals that are represented by palaeosols within the succession. This sequence includes several ignimbrites and surge deposits, debris flow and weathered pumiceous deposits together with fragments of ignimbrite (with fiamme), and basaltic scoria and lithics of diverse origin. Fallout deposits frequently show internal structures that are related to post-depositional slumping/movement on steep surfaces of an older dissected landscape.

This sequence shows great complexity and it is not possible to establish with certainty which deposits were erupted from Fogo. One of the deposits from near the top of the sequence has been dated at 21.34±0.13 ka (Wallenstein 1999). Ponte (2013) established that the deposits from the Chã das Gatas sit on top of this sequence but underlie the Coroa da Mata Formation.

Chã das Gatas eruptive sequence

This small unit was observed near Ribeirinha village, resting directly over the Pico das Freiras basaltic scoria cone. It corresponds to a pile of volcaniclastic deposits, mainly composed of several pyroclastic deposits, generally much weathered with lapilli and ash interbedded with basaltic scoria deposits and palaeosols. This unit is divided by Ponte (2013) into two members, the Calhau do Cabo (lower) and Defeira (upper), separated by a hydromagmatic tuff of basaltic nature, earlier described by Wallenstein (1999) and Pimentel (2004), that can be observed lying on the upper part of the lower member (Ponte 2013). There is also a compact and homogeneous ignimbrite with gritty ash and dispersed lithic clasts with syenitic xenoliths distributed throughout the outcrop.

Coroa da Mata Formation

This formation corresponds to the deposits of a single eruption that occurred at 18.6±0.3 ka (Moore & Rubin 1991), approximately 2.5 km SE of Ribeirinha Village. Probably owing to vent migration, the eruption built up a double pumice cone that surrounds a trachytic dome that formed during the last phases of activity. A similar dome-forming process is described for the 1439–43 and 1630 eruptions of Furnas (Queiroz et al. 1995; Guest et al. 1999, 2015). The proximal deposit is dominated by coarse pumice blocks that reach 30 cm in diameter and is rich in lithics, with boulders ranging in size to >50 cm. Obsidian fragments are abundant. The deposit shows some internal stratification and alternating coarser and finer pumiceous layers. The pumice is rich in feldspar crystals and some amphibole (Wallenstein 1999).

Barrosa eruptive sequence

This is a sequence of deposits in the Barrosa area about 3.5 km north of Ribeira Grande and mainly consists of weathered material, separated by several palaeosols and some erosional surfaces. The unit is limited at its top by a pumiceous breccia, with some basaltic lithics and by a thick pumice-rich ignimbrite with an ashy matrix that reaches >15 m in one of the observed outcrops. No stratigraphic correlation was possible with other units.

Fenais da Luz eruptive sequence

This unit crops out between Fenais da Luz, approximately 10 km west of the town of Ribeira Grande, and the beach at Santa Bárbara on the western vicinity of Ribeira Seca village. This unit corresponds to a sequence of pyroclastic deposits, identified in the sea cliff near to the village of Fenais da Luz. The unit is dominated by two major pumice lapilli fall deposits – both with a maximum thickness of >4 m – that are separated by several thin lapilli and pumiceous ash deposits, a number of palaeosols and a fine basaltic scoria layer. These two major lapilli fall deposits represent two major explosive eruptions. The deposits are lithic rich, showing good sorting of medium to coarse white and fibrous lapilli pumice. The lower deposit contains syenite xenoliths. The upper deposit is crystal poor and some of the pumice clasts show evidence of magma-mingling textures.

Southern Flank

The Roída da Praia Formation defines the base of the Upper Group on the southern flank).

Roída da Praia Formation

This impressive unit is exposed on the southern flank, along the sea cliffs and the major valleys between Ribeira Chã and Vila Franca do Campo. It defines the base of the Upper Group on the southern flank and it is well exposed in sea cliffs at Roída da Praia beach and in a road cut at Pisão. Booth et al. (1978) referred to this unit as a 95 m sequence mainly composed of pumice fall deposits and they recognized 65 eruptive events, of which five were of high magnitude. This formation directly overlies a feldspar-rich lava flow that was dated by Moore & Rubin (1991) at >40 ka.

The Lower Member begins with a series of alternating pumiceous and lapilli deposits, some showing coarser clasts, with ash layers and palaeosols (Fig. 8.7). Around the middle of the sequence, a homogeneous ashy ignimbrite rich in sub-centimetre lithics, with an age of 34.2 ka (Shotton & Williams 1973, in Booth et al. 1978), can be observed filling a depression and passing to a veneer over a distance of a few metres.

The Intermediate Member begins with a layer of homogeneous ash of grey colour and sits directly on a carbon-rich soil from which charcoal fragments were dated at 14.62±0.06 ka BP (Wallenstein 1999). A fine lapilli layer showing some reverse grading, with a pale-colour thin ash deposit on top, marks the transition to a more explosive event. A medium to coarse white pumiceous lapilli deposit, with some big blocks, occurs at this point in the succession. In one of the outcrops, a block of pumice fused some pumice lapilli clasts around it. The deposit also contains many syenite xenoliths that show skins of geothermal alteration. On top of this deposit rests an ash with two layers of lapilli pumice and soil.

The Upper Member comprises mainly an alternating pumiceous lapilli and ash deposit. However, the presence of an important epiclastic deposit rich in lithic blocks provides evidence of important erosional processes on the flanks of Fogo during this period.

It was not possible to establish any correlation of the deposits from this Formation with the pre-Ribeira Chã formations on the northern flank. In the southern flank, a thick and well-developed soil frequently covering lahars marks the separation between this unit and the base of Ribeira Chã Formation, indicating that a major time interval occurred between the two formations.

Ribeira Chã Formation

This formation is composed of deposits from a single paroxysmal eruption. It was initially characterized by Booth et al. (1978), Moore (1990, 1991b) and Wallenstein (1999) on the southern flank. On the northern flank, Pimentel (2004) identified a deposit in a new road cutting near to Santa Bárbara that shows some of the distinctive characteristics of the Ribeira Chã described in detail by Wallenstein (1999) from southern flank exposures. Further work needs to be carried out to confirm this putative correlation.

On the southern flank the Ribeira Chã Formation may be observed from Lagoa to Vila Franca do Campo, with good outcrops (Fig. 8.8) occurring within sea cliffs, in steep valleys and in road cuttings. Wallenstein (1999) divided the Ribeira Chã Formation into two sub-units (i.e. the Lower and Upper members) that correspond, respectively, to the pumice fall and pyroclastic flow deposits produced during a violent paroxysmal but short-lived eruption (Fig. 8.9). The pumice fall sits immediately on top of a well-developed palaeosol, and charcoal from this allowed Moore & Rubin (1991) to obtain two dates of 15.180±150 and 15.19±0.28 ka BP. Wallenstein (1999) also obtained an age for the same level of 14.82±0.060 ka BP, which is older than the date obtained for the Roída da Praia Formation. One explanation for this discrepancy could be that the charcoal samples used to date the deposit were collected from a fault-controlled valley, where juvenile CO2 emission could result in the radiocarbon dating giving anomalously old ages, as has been observed for Kilauea (Rubin et al. 1987) and for Furnas (Guest et al. 1999; Pasquier-Cardin 1999). Based on the stratigraphy and the development of palaeosols, Booth et al. (1978) estimated an age of approximately 7 ka for the Ribeira Chã Formation. Assuming that the main Fogo eruptions show a frequency of 5–10 ka, Widom et al. (1992) considered that the trachytes produced during the eruptions of Ribeira Chã, Fogo A and Fogo 1563 evolved from a common parental magma and that each of these eruptions represented the end of a cycle within a differentiating reservoir. Based on the application of the uranium isotopic series disequilibrium age model to the syenite xenoliths in those deposits, Widom et al. (1993) managed to divide them into three groups of (1) <10 ka; (2) 12–16 ka and (3) ≥200 ka, considering them the plutonic equivalents of the trachytes of Fogo A, Ribeira Chã and the Pico da Eira Velha dome. Based on this and on stratigraphic evidence, Wallenstein (1999) argued for an age of 8–12 ka for the Ribeira da Chã Formation. On the lower southern flank, the Ribeira da Chã Formation's Lower Member is >3 m thick, with some outcrops being >5 m. The pumice layer displays clear clast support and generally shows good sorting, although some of the larger blocks frequently reach >60 cm in diameter (Fig. 8.10). The pumice is white, has low vesicularity and has few feldspar crystals although some amphibole and biotite are found. The lithics sometimes reach >50 cm in diameter in sea cliff exposures that lie >3 km from the vent. A very distinctive characteristic of this deposit is the presence of syenitic xenoliths. A sharp transition to the Upper Member is observed in almost every outcrop on the southern flank of the volcano (Fig. 8.11). The ignimbrite is an excellent stratigraphic marker, owing to the presence of a distinctive welded facies near to its base. In many palaeovalleys on the lower flank, between Ribeira Chã and Água d'Alto, the ignimbrite shows a considerable variation in thickness between the valleys, where it may be >10 m in thickness and shows no evidence of rheomorphism. On interfluves the formation is frequently very thin or even absent. Where the welded facies is less well developed, a non-welded ignimbrite separates the welded facies from the fall deposits. The non-welded facies at the top shows an ashy matrix of pinkish-brown colour, rich in sub-centimetre-sized lithics. Some levels show a high concentration of coarser lithics and others of coarse and frequently banded pumice. This indicates the presence of several flow units in the same cooling body of the ignimbrite.

Detail of the sharp transition between the pumice fallout and welded ignimbrite from the Ribeira Chã Formation.

Upper Group, post-5 ka

This section considers the activity of Fogo over the last 5 ka, which includes the important Fogo A and 1563 (historic) eruptions. This period is important in providing the context for the current behaviour of Fogo and the hazard it presents.

Pisão Formation

A basal vesicle-rich ash is overlain by a homogeneous medium-grained and poorly sorted white to yellowish lapilli pumice, which represents the deposits from a single eruptive event that rests over the mudflows that cover Ribeira Chã. It is observed on the southern flank and is almost always associated with and immediately below Fogo A materials. It does not vary much in thickness across the observed outcrops that are present in sea cliffs. The charcoal found on top of basal ash is dated by Wallenstein (1999) at 4.76±0.06 ka BP. It is separated from Fogo A by a poorly developed palaeosol.

Fogo A was first described in detail by Walker & Croasdale (1971). It comprises two major fallout deposits, a lower syenite-poor pumice fall and an upper syenite-rich lapilli fall, separated by a pyroclatic surge (or pyroclastic density current – PDC) deposit. Walker & Croasdale (1971) produced isopach and isopleth maps for the whole deposit. Bursik et al. (1992) determined separate isopach maps for each fallout deposit, but with much smaller datasets than those used by Walker & Croasdale (1971). Wallenstein (1999) provided a detailed description of each unit within the formation and Engwell et al. (2013) discussed and quantified the uncertainties associated with thickness measurements.

Fogo A is the best stratigraphic marker horizon for the whole island, having been used in stratigraphic studies of other volcanoes, for example, Furnas (Guest et al. 1999), Sete Cidades (Queiroz 1997; Queiroz et al. 2008) and the Picos Fissural Volcanic System (Ferreira 2000). There are references in previous works to several radiometric dates obtained from charcoal dispersed in different layers of the deposit – 4.673±0.03 ka BP (Shotton et al. 1968, 1969 in Walker & Croasdale 1971), 4.55 ka BP (Booth et al. 1978); 4.48±0.08 to 5.38±0.21 ka BP (Moore & Rubin 1991) and 4.23±0.15 to 4.52±0.09 ka BP (Wallenstein 1999) – and it is commonly assumed that the eruption dates from around 4.6 ka BP. Wallenstein (1999) divided Fogo A into two members, Lower and Upper (Fig. 8.12), with the transition being marked by the pyroclastic surge deposit (Fig. 8.13; see above), which coincides with the boundary proposed by Bursik et al. (1992) based on the syenitic xenolith content.

General view of Fogo A on the sea cliff of the southern flank: (a) Lower Member; (b) Upper Member.

The Lower Member (Fig. 8.14) has a layer of compact dark grey ash (C1) at its base that typically shows hydrothermal alteration and rests directly on a carbon-rich soil, in which well-preserved charcoal fragments occur. It is frequently absent and in its place a very thin layer (occasionally up to a few centimetres) of fine pumice lapilli (L1) was observed. It has a lithic-rich base, and in the absence of C1 and when very thin, the pumice is mixed with the organic soil material. However in a few outcrops it can reach almost 10 cm. A second layer of compact dark grey ash (C2) is observed over the previous sequence. As for C1, this ash is of hydromagmatic origin and includes vesicles and occasional accretionary lapilli, and in some outcrops it is possible to observe moulds of small trunks and leaves. At the top of this sequence, there is a pumice lapilli layer (L2) rich in feldspar phenocrysts and sub-centimetre lithics, a light grey to white ash (C3) with vesicles; a pumice lapilli horizon (L3) similar to L2 and always very thin and, finally, a layer of light grey to white ash (C4). Not all beds are present in all outcrops and layers C1 and C4 are most commonly absent from the sequence C1–L1–C2–L2–C3–L3–C4.

The pumice fallout deposit (L4) is the most characteristic and impressive of Fogo A. It can be identified in almost all outcrops and is frequently the only one that is present in distal outcrops >10–15 km from the vent. It is possible to distinguish, mainly in exposures on the southern flank and over a radius of around 5–6 km from the vent, an internal stratification with a lower part (L4b) being less coarse than the upper part (L4t). It is a relatively well-marked transition that indicates an increase in the eruptive column height. The deposit has a very clear clast-supported sedimentology, with loose white pumice, that is both very homogeneous and vesicular. Lithics of diverse petrology are present, including many with skins showing geothermal alteration. One of the most characteristic aspects of Fogo A is the abundance of loose feldspar (sanidine) crystals in the sub-2 mm grain size distribution, which is of great help in field identification of this deposit, especially in more distal locations. At its top, Fogo A shows a bimodal grainy ash bed (C5) with dispersed rounded pumice lapilli clasts. This may represent a mixture of a pumice fall deposit with the distal portions of a PDC that originated from partial column collapse. A PDC deposit (S1) with frequent cross-bedding structures and variable thickness is normally observed on the top of L5 and marks the transition to the Fogo A Upper Member.

The deposits of the Upper Member mark the beginning of the end of the Fogo A eruption, with alternating pumice fallout deposits and bimodal ashes that represent distal intra-Plinian pyroclastic flow deposits (Walker pers. comm.; Hayashi-Smith 1992; Wallenstein 1999). The sequence ends with a major ignimbrite that covers a large area of the volcano's flanks, with accumulations of up to >20 m in some valleys. The variability in thickness of the proximal and distal flow deposits makes it impractical to correlate all of the individual deposits (Hayashi-Smith 1992), but Wallenstein (1999) identified three main fall horizons: L6, L7 and L8; two bimodal ash beds (PDC) deposits (i.e. C6, C7) that may be correlated between outcrops, mainly on the southern flank; and the ubiquitous top ignimbrite (It), which frequently comprises several flow units with a clast distribution similar to level 2 of the proposed model of Sparks et al. (1973). The lapilli fall deposits, L6 and L7, show medium to coarse pumice lapilli and are also rich in feldspar phenocrysts. Many of the pumice clasts are banded, alternating light and dark colours, and clasts of dark pumice increase in frequency towards the end of the eruption. The syenitic xenolith content also increases from the base to the top (Bursik et al. 1992; Wallenstein 1999). The ignimbrite (It) is, in general, brown to dark brown with brown-pinkish colour variations in the deeper depressions. It is composed mainly of a poorly sorted fine ash matrix, with pumice clasts and lithics similar to the ones of the preceding deposits (Hayashi-Smith 1992; Wallenstein 1999). On the northern flank charcoal fragments and concentrations of lithics with internal structures similar to fluvial channels were observed in several outcrops and a few degassing pipes were identified (Wallenstein 1999).

Lombadas Formation

This formation was defined by Wallenstein (1999) and is represented by a sequence of three trachytic pyroclastic deposits described by Booth et al. (1978) as Fogo B, C and D that were produced in a time span of <3 ka. Although more recent than Fogo A, the low volume and dispersion of the deposits, mainly concentrated in the higher and deeply eroded areas of the volcano edifice, did not allow any significant improvement to the isopach maps of Booth et al. (1978). Fogo B is dated at 3.242 ka BP (Shotton et al. 1970 in Booth et al. 1978) and is distinctive because it has its vent outside the Summit Caldera, about 3.1 km SSW of the centre of the Fogo lake at Mata do Botelho. Its pyroclastics are dispersed mainly to the north and to the east (Booth et al. 1978). Although it was not possible to observe the thickness of the total deposit in proximal areas, sections 5 m thick were documented and Booth et al. (1978) describe outcrops some 10 m thick. The base of the deposit is composed of compact fine ash with several beds of sandy and fine lapilli pumiceous clasts that cannot be correlated between nearby outcrops. The beds increase in thickness towards the top of the deposit, where the lapilli fall deposits become dominant, just below a final ash layer. One distinctive feature of these deposits is that they are rich in loose feldspar crystals in a manner similar to Fogo A, but they are much richer in lithics and this, together with the absence of syenite xenoliths, makes them easy to recognize. The top of this deposit shows a very irregular surface with evidence of intense erosion, with a clear time interval before the deposition of Fogo C deposit. Fogo C is weathered and is always limited by erosional surfaces and, therefore, does not permit the whole deposit to be examined in a single outcrop. The deposit is composed of homogeneous ash of yellowish or light brown colour. Booth et al. (1978) refer to the existence of frequent accretionary lapilli. The Fogo D deposit is well stratified, particularly at its base, alternating fine and medium clasts layers with ash beds. The upper part is composed mainly of yellowish-grey ash with some dispersed pumice clasts.

Basaltic flank eruptions

Pre-Fogo A scoria cones and associated lava flows

As already discussed, the Fogo A deposit provides a distinctive marker horizon that enables scoria cones that predate this eruption to be readily identified in the field. Further stratigraphic constraints can be placed on the age of the Pico das Freiras cone whose scoria underlies the deposits of the Chã das Gatas Formation and the two cones E of Lagoa, which are older than the Ribeira Chã Formation. The Monte Gordo cone, the lava flows of which lie immediately below the Fogo A fallout deposit, was dated by Moore & Rubin (1991) at 4.99±0.1 ka BP.

Post-Fogo A scoria cones and associated lava flows

Pico Arde (northern flank) and Pico de Nossa Senhora das Angústias (southern flank near Água de Pau) were formed by pre-historic Post-Fogo A eruptions on the flanks of Fogo. The Pico Arde scoria cone was formed at 1.790±0.150 ka BP (Moore & Rubin 1991), is located about 700 m south of Pico Vermelho and is poorly preserved. However, basaltic scoria and spatter can still be observed, which show a porphyritic texture with feldspar phenocrysts. A lava flow was erupted towards the northern coast, where it forms the lava delta that separates the Ribeira Grande and Santa Bárbara beaches (Wallenstein 1999). Along its path towards the sea a lava tube, known as Gruta do Esqueleto (Constância et al. 1994), is found. The Pico de Nossa Senhora das Angústias scoria cone erupted lavas that formed part of the lava delta of Caloura. The basalt is dark grey with an olivine-rich porphyritic texture and contains syenite xenoliths. Older parts of the lava delta are mantled by Fogo A lapilli.

Historical eruptions

There are historical records of two volcanic eruptions of Fogo in 1563 and of a probable phreatic explosion in 1564. The 1563 eruptions started 4 days apart. The first, of sub-Plinian character occurred on 28 June inside the Summit Caldera. The second, of Hawaiian style, took place on 2 July on the northern flank of the volcano. The 1564 phreatic explosion occurred on 10 February approximately in the same location as the 1563 explosive eruption (Wallenstein 1999; Wallenstein et al. 2005, 2007).

1563 sub-Plinian eruption

The deposits of this single eruptive event are described in detail by Walker & Croasdale (1971). Their impressive isopach and isopleth maps confirm the historical records and attest to the existence of winds from WSW that led to a very asymmetrical dispersion of the lapilli and ash, with the main axis towards the ENE and covering almost all of that quadrant of the island towards Nordeste village, approximately 30 km distant from the eruptive centre. This event was the largest since Fogo A, however, with only one-quarter of the estimated volume of ejected tephra (Walker & Croasdale 1971). Wallenstein (1999) also studied this deposit, which is well stratified with the basal part being dominated by hydromagmatic fine ashes with interbedded layers of fine lapilli that increase in frequency and thickness towards the top of the deposit where the lapilli component is dominant (Fig. 8.15). The pumice is of white colour and crystal poor. The finer layers are lithic rich. Some syenitic lithics are observed but are much less abundant than in Fogo A.

1563 Pico do Sapateiro

The source of this Hawaiian-style single eruptive event was on the trachytic dome of Pico do Sapateiro, renamed Pico Queimado after the eruption. The basaltic magma ascended along a NW–SE fault that crosses the dome and fed a lava fountain that generated scoria, bombs and spatter, and which accumulated around the vent. Two lava flows were erupted. The basaltic lava, dark grey in colour, has a porphyritic texture with abundant phenocrysts of olivine in a matrix of plagioclase, pyroxene, olivine and opaque minerals. The lava is rich in xenoliths and xenocrysts derived from syenite and lesser amounts of partially fused trachyte (Wallenstein et al. 1998).

Petrology

The basaltic rocks sampled by the Dalhousie borehole (Muecke et al. 1974; McGraw 1976) showed that, in common with the other volcanic systems of the island, the earliest eruptions of Fogo were basaltic. The 81 samples collected by Wallenstein (1999) to reflect the stratigraphy of the volcano included several rock types, including syenite xenoliths. This research studied only trachytes from the Lower Group together with a range of samples from the Upper Group. The rocks from Fogo Volcano define an alkaline series (Middlemost 1997) with a potassic signature (Na2O2<K2O), ranging from basalt to trachyte. Intermediate members include potassic trachybasalts, shoshonites and latites (Fig. 8.16), showing the ‘Daly Gap’ (Daly 1925), in this case characterized by an absence of samples in the SiO2 58–62% range as well as in that of MgO 5.5–9%. This is a feature that is present in other rock suites from the Azores (e.g. Self & Gunn 1976; White et al. 1979; Queiroz 1997). The study also included a basanite sample, collected from the scoria deposit at Pico das Freiras. Many of the trachytes sampled by Wallenstein (1999) show a peralkalinity index >1, with all samples falling in the field of comenditic trachytes. Major elements show behaviour typical of a magmatic series in the total alkali v. silica diagram (Fig. 8.16).

The variation of the relative quantities of certain elements in some sequences of Fogo's recent deposits has been studied by several authors (Storey 1981, 1982; Widom et al. 1992). Storey (1981, 1982) analysed samples collected by George Walker's team from Fogo A–D and 1563 deposits, finding significant variations in the chemistry and mineralogy of pumice clasts from the different deposits. They were interpreted as successive samples from a magma body of trachytic nature undergoing a process of fractional crystallization. Based on this finding, Storey et al. (1989) proposed the existence of a chemically zoned magma chamber. The presence of shallow magma reservoirs, where basaltic magma originating from deeper sources is differentiated, generating the gas-rich trachytic liquids that feed more explosive eruptions, was proposed for Fogo (Marriner et al. 1982; Wallenstein 1999). Basalt magma that does not intercept a high-level reservoir can rise directly to the surface, feeding the basaltic flank eruptions. Trachytic and basaltic eruptions can occur almost simultaneously, as happened during the 1563 eruptions (Wallenstein 1999). It is possible that an ascending pulse of basaltic magma reached the trachytic reservoir, triggering the explosive eruption with some of the basaltic magma being injected beyond the margin of the reservoir along the fault that cuts the Pico do Sapateiro dome, feeding the subsequent basaltic eruption. The trachytic magma reservoir appears to act as a filter preventing eruption of basaltic magma in the central zone of the volcano (Fig. 8.4).

Widom et al. (1992) restricted analysis to different levels of the Fogo A deposits, studying the variation in the mineralogy and chemistry of the whole rock, as well as 25 samples of phenocrysts. These authors then compared the results with those from samples of the Ribeira Chã Formation (designated as Fogo C in their work) and Fogo 1563. They concluded that the trachytes from these three eruptions evolved until a certain degree of fractionation from a common alkaline basalt parental magma generated through partial melting of a mantle source. Wallenstein (1999) collected 36 sequential samples from 28 deposits of the Roída da Praia, Ribeira Chã, Pisão, Fogo A, Lombadas and 1563 units (Fig. 8.17). More detailed sampling was performed on Fogo A, with nine samples. The oldest sample was collected on the 34.2 ka ignimbrite and the youngest from the lapilli of the 1563 deposit. All the samples were of trachytic composition, showing a peralkaline character. Variations did not show a clear pattern for all of the sequence, but for Fogo A it was evident that the decrease in degree in the differentiation – from the bottom to the top – reflected an emptying process of a chemically zoned magma reservoir. It was also observed that the existence of banded and dark pumice towards the top of the deposit was due to mixing of magma from different evolutionary stages. Inflection in the lines of chemical variation of the level L8 is a good expression of this phenomenon because the analysed samples are of light (the lower) and dark (the upper) colour, a consequence of their chemical compositional differences. The sample collected from the ash of the top ignimbrite shows a composition that represents average values of the two samples of L8, which suggests the mixture of particles in the column immediately before its collapse.

Eruptive history

The submarine edifice

The accumulation of the materials that form the base of the Fogo massif erupted >200 ka ago, taking into account the age determination of the submarine lavas collected at a depth of 786 m below sea-level (Muecke et al. 1974). Fogo rises from the Azores Plateau (Needham & Francheteau 1974) at 2000 m below sea-level and, disregarding erosion, it may be concluded that the construct has a height of >3500 m (Wallenstein 1999). The growth of the submarine pile resulted from the accumulation of basaltic lava flows building up a shield volcano. This period is probably broadly contemporaneous with similar phases of volcanic evolution on Furnas and Sete Cidades volcanoes (Queiroz 1997; Guest et al. 1999), and was classified as a subaqueous sequence (Muecke et al. 1974; McGraw 1976), in which many pillow-lavas with vitreous surfaces occur, and this was followed by a sequence that marked the transition to the subaerial volcanism.

Variation (%) of the major elements in the Fogo eruptive frequency. See text.

The subaerial activity

Older subaerial activity was described by Muecke et al. (1974) as being represented by three extensive sequences composed predominantly of basaltic lava flows, interbedded with thick pyroclastic units. This represents the earliest evidence of differentiated magma reservoirs as indicated by the presence of lavas and pyroclasts of trachytic composition. The presence of the trachytic lava dome of Pico da Eira Velha located on the southern coast and dated at 181 ka and several trachytic lava flows and domes with ages ranging around 120 ka occurring at both low and higher altitudes on the northern flank of the volcano may indicate the presence of more than one differentiated magma reservoir in the early stages, but later the magma reservoir became centralized, leading to the development of the central volcano. Muecke et al. (1974) stated that the period of the most intense activity was leading up to 100 ka BP, represented by major ignimbrites interbedded with trachyte lavas. However, Gandino et al. (1985) refer to the occurrence of several paroxysmal eruptive events for the period between around 86 and 74 ka. Based on geomorphological evidence of ridges on the northern margin of the Lombadas valley, Moore (1991a) proposed the existence of the Outer Caldera, and associated this with an ignimbrite deposit erupted between approximately 26 and 46 ka. This was contemporaneous with a submarine tephra deposit found on the NE of the island that was dated by Huang et al. (1979 in Moore 1991a). Although there is no clear stratigraphic evidence to link the ignimbrite with the caldera, we agree that, although much modified by erosion, geomorphological and structural evidence both support the presence of a caldera.

The last 40 ka have been characterized by bimodal volcanic activity comprising basaltic flank eruptions and trachytic explosive activity from the summit area. It may be argued that a first phase of caldera fill occurred with explosive eruptions being responsible for the deposits of the Roída da Praia Formation and some of those of the Porto Formoso eruptive sequence. Clear relationships, however, between the deposits and collapse and fill landforms cannot be established. Some trachytic explosive and dome-forming events were also produced during this period, as exemplified by the Coroa da Mata Formation.

Summit Caldera-forming eruptions

The Ribeira Chã Formation is made up of the deposits of a single paroxysmal eruption. The internal structure and dispersion parameters of the deposit allowed Wallenstein (1999) to interpret this as a short-lived, violent eruption that occurred between 8 and 12 ka BP. It was centred on the volcano summit and was responsible for the development of a Plinian eruptive column that collapsed abruptly. It is likely that the rapid explosive eruption of a large volume of magma from the trachytic reservoir produced caldera collapse at the summit, leading to the formation of a very characteristic ignimbrite deposit, which is frequently welded when cropping out in some palaeovalleys. The rapid accumulation of loose materials and strong precipitation on the steep flanks of Fogo allowed the further development of lahars.

After a long dormant period characterized by the occurrence of mudflows and the formation of soils, eruptive activity at the summit recommenced around 4.8 ka ago with a moderate trachytic explosive eruption that produced the deposits of Pisão Formation. It is of note that this eruption was almost immediately preceded by the basaltic flank eruption of Monte Gordo around 5 ka BP, which fed a lava flow that reached the sea near Ribeirinha.

Very close in time, around 200 years later, as verified by radiocarbon dating and a very thin palaeosol, the Fogo A eruption occurred. It was a Plinian event that also played a very important role in forming the shape of Fogo's Summit Caldera. The eruption started with a few small magmatic explosions and hydromagmatic episodes that show intermittent contact of magma with water; a steady and stronger Plinian-type eruptive column produced a pumice fallout deposit, as recognized by Walker & Croasdale (1971). The dispersion axis was to the south (Bursik et al. 1992). During this phase thunderstorms occurred, as evidenced by fulgurite structures within the deposit (George Walker pers. comm.; Wallenstein 1999). An obstruction in the vent marks a clear transition to a higher eruptive column within the late phases of this fall episode. Then, a small lateral/partial column collapse, probably owing to the widening of the vent, indicates the beginning of variations in the column stability and, after another fallout episode, a more widespread PDC deposit marks the occurrence of another column collapse.

In the next phase of the eruption the lower levels and the walls of the zoned magma reservoir were being reached, tapping a mixture of more mafic magmas. A more unstable or pulsating eruptive column produced the distinctive deposits described as being syenite-rich by Bursik et al. (1992) and marking the final phase of this eruption. It corresponds to the Upper Member of the stratigraphy of Wallenstein (1999), and was generated by a sequence of alternations of column height, caused by explosions followed by collapse, as the gas content of the magma decreased. This was a process in which the collapses periodically obstructed the vent and alternated with increases in magma pressure in the conduit, which led to new explosions (Carey & Sigurdsson 1989). During this phase of the dispersion the fallout of denser material from the plume was little affected by wind and this resulted in the tephra being concentrically distributed around the centre of the Fogo Summit Caldera. The final collapse marks the end of the eruption, with the production of massive pyroclastic flows and ignimbrite deposits covering most of the volcano's flanks. Its thickness is >20 m in the main valleys and depressions, in particular in the Ribeira Grande graben.

Post-Fogo A volcanism

The three explosive eruptions of trachytic nature, Fogo B–D (Booth et al. 1978), were of much lower magnitude than the earlier Fogo A. The first took place at 3.2 ka BP and was the only one of the three that was centred outside the Summit Caldera, producing a crater that cuts part of the Mata do Botelho trachytic dome (Wallenstein 1999). This eruption started with a small and unstable eruptive column, in some places flushed by rain (Booth et al. 1978), followed by an increase in magnitude that produced a lithic-rich fallout deposit and ending with a collapse that generated surges. The dispersion of this deposit was studied by Booth et al. (1978), who suggested the existence of moderate to strong winds from the SW. The existence of well-developed palaeosols and important erosional surfaces indicates a period of time between the deposition of Fogo B and Fogo C. Extensive weathering of the deposits means that it is not possible to describe in detail the eruptive episodes of Fogo C and D. From what can be observed in the deposits, Fogo C was essentially hydromagmatic, whereas Fogo D involved mainly magmatic explosions with limited hydromagmatic activity. According to dispersion studies carried out by Booth et al. (1978), the eruptive centres of both eruptions were located within the Summit Caldera. The Booth et al. (1978) Fogo C MP and ML isopleths, the deposits characteristics and analysis of terrain, led to the conclusion that the crater located on the SE sector of the lake should be attributed to Fogo C, while the pumice cone north of the lake would have been formed by Fogo D. Both structures were then covered by ash and lapilli from the 1563 intracaldera eruption. Owing to the limited dispersion of their products, it is not possible to correlate the basaltic flank eruptive activity with the more explosive events of the summit area. Based on radiocarbon dating, however, the Pico Arde eruption occurred much later than Fogo B.

Historical volcanism

One of the challenges of studying historical volcanism is to integrate written historical records with physical evidence collected in the field. In the case of Fogo the excellent historical records from the sixteenth century, such as Frutuoso ([1522–91†] 1977) among others, allowed the description and interpretation of the chronology of the 1563 eruptions and this is supported by field evidence (Wallenstein et al. 1998, 2007; Wallenstein 1999).

On 23 June 1563, intense seismic activity began to be felt by the population of several localities on the island, causing significant damage and destroying many buildings in Ribeira Seca and Ribeira Grande. On Monday, 28 June, 1 h before sunset, inside the Fogo Summit Caldera and in the middle of the lake, the activity started with loud explosions that illuminated the sky. The dark eruptive cloud could be seen on the next day only from a small crater lake on the top of Pico das Berlengas (Frutuoso [1522–91†] 1977), also known as Pico da Lagoinha, which according to a priest named Bayão (Canto 1878), was surrounded by a larger lake. During the first night, WSW winds meant that ashes and lapilli were deposited on the eastern and northeastern parts of the island (Frutuoso [1522–91†] 1977), as illustrated by the isopach maps of Walker & Croasdale (1971). On Thursday, 29 June there was a slight change in wind direction to the NE quadrant, causing 10 cm of ash to fall on Ponta Delgada (Frutuoso [1522–91†] 1977). During the next 3 days the ash continued to fall at Ribeira Grande, Porto Formoso and Maia villages and even further east in Achada, São Pedro Nordestinho, Furnas, Povoação and Nordeste villages. In addition to the ash, falls of pumice and lithics were also reported by Frutuoso, as well some ash fall in Vila Franca do Campo. Landslides and mudflows travelling along Praia river bed also reached the southern coast in the area near Água d'Alto.

The eruption was accompanied by strong seismicity until 1 July and eruptive activity ended on Saturday, 3 July, 1 day after the beginning of the Pico do Sapateiro flank eruption. This historic flank eruption took place on Friday, 2 July, 4 days after the beginning and 1 day before the end of the intracaldera eruptive activity. The opening of the vents occurred on a NW–SE fault that crosses the trachytic dome known as Pico Queimado after the eruption. The eruption began with the ejection of what can be interpreted from Frutuoso's description as ballistic blocks and bombs of basaltic composition. The eruption column was described as being dark. Two days later there was an increase in activity and, according to Frutuoso, lava began to flow in the direction of the coast (Fig. 8.18). It followed the Vilão and Ribeira Seca river beds, overflowing the latter near to the southern limits of the village of Ribeira Seca 2 days later. It reached the sea at the Santa Bárbara beach (Fig. 8.19) on 7 July, after flowing along the margin of the Pico Arde lava flow (Wallenstein 1999). A second eruptive vent started to feed another lava flow that developed towards the NW. It covered some wheat fields and finished its progress abutting the lava flow of Mata das Feiticeiras (dated at about 1 ka; Moore & Rubin 1991; Fig. 8.3). There are no records of when this eruption ended, but there are historical records of its activity on 28 July.

Frutuoso ([1522–91†] 1977) records the occurrence of a big explosion on 10 February 1564, near the place where the 1563 eruption started on 23 June, on Pico das Berlengas; this opened a big grotto (algar) from which thick smoke emission could be observed and a ‘liqueur’ flowed down the mountain towards the south. The quality of the reports with regard to the events of 1563 gives confidence in taking seriously this written record of an event in 1564. Unfortunately no deposits have been identified from this eruption. However, it is possible that a younger scar in the 1563 eruption crater wall is related to this event. It is probable that this was a phreatic eruption caused by the contact of water with the still hot body emplaced during the 1563 eruption. The ‘liqueur’ referred to by Frutuoso ([1522–91†] 1977) could be the development of small lahars whose deposits have subsequently been removed by erosion.

Final remarks

Of the three active central volcanoes of São Miguel island, Fogo covers the most extensive area and presents the most complex morphology. This morphological complexity mainly occurs in the summit area, where its calderas and craters are not well defined when compared with Sete Cidades and Furnas. Understanding the detailed evolution of Fogo is, therefore, more difficult.

Fogo presents an impressive record of its eruptive history, since the earliest stages of transition from submarine to the subaerial activity and generated at least four deposits that relate to important paroxysmal events, including extensive tephra fall deposits and ignimbrites. Two of these events, Ribeira Chã and Fogo A, occurred, respectively, 8–12 and 4.6 ka before present and marked the most recent evolution of the Summit Caldera. Fogo A deposits crop out extensively over most of the island, forming a marker horizon that facilitates stratigraphical correlation. Ribeira Chã and Fogo A, together with the 1563/1564 historical eruptions, are valuable in contributing to an understanding of Fogo's plumbing system (Storey et al. 1989; Widom et al. 1992) and the establishment of future eruptive scenarios for hazard and risk assessment (Wallenstein et al. 2007).

Lahars, landslides and seismic activity are also recurring events in Fogo, as shown by suites of epiclastic deposits. Historical records reveal that the most destructive geological event to have occurred since the Azores were settled was the earthquake and associated landslides that struck São Miguel on 22 October 1522 (e.g. Machado 1966; Silveira et al. 2003; Marques 2004; Wallenstein et al. 2005, 2007).

Acknowledgments

We should like to thank David Chester for the very fruitful discussions and text revision, which allowed considerable improvements in the final work. Catarina Goulart and José Medeiros gave valuable support in digital elevation model and graphics. Pedro Sousa kindly provided the photo in Figure 8.2. Constructive comments from Paul Cole and Gabriela Queiroz improved the manuscript.